1. Field of the Invention
This invention relates to electronic circuitry for actuating fuel injection valves in timed relation to the operation of the engine, and more particularly to a means to correct for incidental variable to the fuel injection system. This invention may be used in a system such as that described in my U.S. patent application Ser. No. 629,421, entitled "Fuel Injection System" and in my U.S. Pat. No. 4,058,709 issued Nov. 15, 1977, entitled "Control Computer for Fuel Injection System," both filed concurrently herewith.
2. Prior Art
Fuel injection systems, employing electrically actuated injector valves to provide metered quantities of fuel to the individual engine cylinders, were originally proposed as devices for improving an engine's performance. In recent years, these systems have received renewed interest because of the prospect of minimizing the polluting components of the engine's emissions. By providing each cylinder with a more precise quantity of fuel as determined by measurements of engine operating parameters, they insure a more complete combustion of the fuel charge. This metering requires a measurement of a variety of engine operating parameters including manifold pressure, engine temperature, engine speed, barometric pressure and the like. Relatively complex "computers" have been developed which receive all of these measurements and adjust the width of an actuating pulse to the coils of the fuel injectors to actuate the injectors for a controlled time to meter the required fuel quantity.
One source of inaccuracy in such systems has resulted from incidental system variables, such as impedance variations of the injector solenoid coils, specific resistance of the wire used in the individual coils of different injectors and the voltage supply to the fuel injection system. The coils are positioned close to the engine. As a result their temperature and hence their resistance will vary between extremes ranging from a low when the engine starts cold in the winter and a high associated with normal engine operation. A temperature range from -20.degree. F. to 300.degree. F. is not unusual for the injector coils. Such a variation in temperature will cause a wide variation in coil resistance.
Previous injector circuits have employed switched outputs which provide a substantially constant voltage source and provide the solenoid coils with current inversely proportional to their resistance. Thus, the current to the coil, and the actuation force of the coil, would vary with the engine temperature. The response time required for the injector to actuate after the start of an actuating pulse is in turn a function of the current applied to the coil. Accordingly, this response time varies with engine temperature and limits the accuracy with which fuel can be metered by the system.